Previous reports have held secretion of one or more toxin(s)
by P. shumwayae and P. piscicida responsible for fish-lesion
outbreaks, fish-kill events, and a suite of human ailments in East Coast
estuaries. However, a Pfiesteria toxin has never been identified or
isolated.

Dinoflagellates are natural and often beneficial components of
aquatic ecosystems. However, a few dinoflagellate species can cause or
contribute to "harmful algal blooms," or
HABs. Most toxins produced by HAB dinoflagellates, including all toxins known to
debilitate or kill fish (ichthyotoxins), are either polyketides or non-ribosomal
peptides.

Polyketides are produced by the PKS family of enzymes.
Reasoning that Pfiesteria would likely contain a PKS gene if it produces
an ichthyotoxin, genetic researchers at Rosenstiel and VIMS tested the
Pfiesteria genome for the presence of PKS genes—without success. "PCR
primers that amplify the polyketide synthase (PKS) genes that produce
polyketides in dinoflagellates do not yield a product with P. shumwayae
DNA," says Dr. Kimberly Reece, a VIMS
molecular geneticist and co-author of the PNAS report.

Tests with NPRS primers likewise failed to amplify the genes
that dinoflagellates use to produce non-ribosomal peptides. However, these
primers did amplify a gene fragment from P. shumwayae DNA that appears similar to the dinoflagellate genes
that synthesize polyketides or fatty acids. "This gene may be involved in
fatty acid biosynthesis, rather than production of a polyketide toxin," says
Reece. "We need to further characterize it and examine its activity to
definitely determine its function; particularly in light of the lack of other evidence for exotoxin production by P.
shumwayae."

While the researchers' work does not entirely eliminate the
possibility that Pfiesteria might produce polyketide toxins, it strongly
suggests that any polyketides the dinoflagellate might produce are either
non-toxic, not secreted into the water, or not released when a dinoflagellate
cell ruptures.